The present invention relates to the magnetic resonance art. It finds particular application in conjunction with gradient coils for magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications which require gradient magnetic fields.
In a magnetic resonance imaging system, the gradient coils are commonly pulsed with current pulses having a short rise time and a high duty cycle. Pulsing the gradient coils produces magnetic field gradients across the imaging region, as well as magnetic field gradients which interact with external metallic structures such as cold shields in a superconducting magnet. This interaction generates eddy currents in the cold shields, which, in turn, generate eddy magnetic fields. The eddy fields have a deleterious effect on the temporal and spatial quality of the magnetic field in the examination region, hence in the resultant image quality.
One approach to circumventing the eddy current problem is to place a shielding coil between the gradient coil and the cold shields. The shielding coils are designed to substantially zero the magnetic field externally of themselves, preventing the formation of eddy currents. However, the shield coil inductively couples with the gradient coil, draws power, and reduces gradient coil efficiency. The required additional current through the gradient coil increases the already high demands on the driving circuit and the power handling capacity of the coils.
More specifically, gradient coils are typically a three-layered coil set that is formed on a cylindrical former. Coils for generating x, y, and z-gradients are insulated from each other and layered on the former. Commonly, the entire assembly is overwrapped and epoxy impregnated for greater structural strength to withstand the warping forces when the current carrying conductors interact with the primary magnetic field.
Various techniques have been employed to derive suitable conductor patterns for the gradient coils. Some gradient coil assemblies use a "distributed" coil design in which the conductors approximate a continuous current distribution function. Other coils begin with a discrete set of conductors which are closely bunched to one another. In both the distributed and bunched coils, the patterns are designed to meet specified electromagnetic design goals, particularly a linearity of the gradients with a minimal energy usage. To achieve these design goals, the radius of each coil is minimized. Minimizing the radius requires placing the coil layers close together in the radial direction. In some designs, the z-gradient coil, which is inherently the most efficient, is placed at the smallest radius. Confining the three coils in such close proximity creates numerous problems in design, fabrication, heat dissipation, and the like.
In order to simplify the system design of gradient shield coils, it is advantageous to drive primary and secondary coils in series. This is often referred to as self-shielding. In self-shielded gradient coils, there are generally two cylindrical coil sets. The larger diameter coil set substantially cancels the magnetic field exterior to itself but interacts with the smaller diameter coil to subtract from the gradient field in the examination region. A mechanical means connects the coil sets into a unitary structure while maintaining the coil sets in a spaced relationship. This type of self-shielded coil again reduces coil efficiency and increases power dissipation.
One of the problems with closely layered x, y, and z-coils is that the currents cause a significant amount of heat in a small confined area. The overwrap and epoxy impregnation resist the transfer of heat from the coil assembly. Although the z-coil is inherently more efficient than the x and y-coil, placing the z-coil as either the innermost or outermost layer fails to take advantage of this greater efficiency from a thermal standpoint. When the z-coil is the innermost coil, it is a heat source closer to the patient bore. Since it is generally layered into machined grooves in the former. Alternately, the innermost placement of the z-coil increases the radius of the already least efficient x and y-coils. When the z-coil is in the outermost layer but physically close the x and y-coils, the z-coil and the epoxy and overwrap add a substantial heat barrier for removing heat from the x and y-gradient coils.
Such a two coil set self-shielded gradient coil is illustrated in U.S. Pat. No. 4,737,716 issued Apr. 12, 1988 to Roemer, et al. The Roemer design approach was to expand the current density stream functions in a suitable Fourier-type series and derive a set of expansion coefficients which yield the desired field gradient linearity and screening/shielding behavior. The described Roemer design is iterative in nature. That is, a winding pattern is designed for the inner coil in a direct fashion. The outer coil is then designed to cancel the exterior magnetic field of the inner coil which, of course, disturbs the linearity of the magnetic field. This requires adjusting the inner coil design to maintain the linearity requirements, which requires adjusting the outer coil design, etc.
one problem with the Roemer method is that it does not consider the inductance or stored magnetic energy in the coil in a direct fashion. This permits the coil design to hold more than a minimal amount of energy, which is energy inefficient and forces one to iteratively search for a solution which is deemed acceptable. Linear, but very inefficient coils can be generated. Further, this technique does not take advantage of the inherent higher efficiency of z-gradient coils.
Another technique for designing self-shielded gradient coils which seeks to minimize inductance or energy storage is described in "Minimum Inductance Coils", R. Turner, J. Phys. E. Sci. Instrum. (19), 1986. Two cylinders which are each assumed to have infinite length support continuous current density functions. Working in the spectral domain, the magnetic field is constrained at a finite number of points with the added constraint that the second cylinder is a superconducting boundary, i.e. the outer coil shields the surrounding structure from the magnetic field gradients. The stored magnetic energy is minimized with these constraints and a direct, analytic solution for the current distribution is obtained. The current distribution is then truncated to account for the finite length of the coils and discretized to produce a practical coil pattern.
One of the disadvantages of the Turner approach is that the coils are assumed to be of infinite length and then truncated. This creates aberrations in the resultant pattern and diminishes the effectiveness of the shielding, particularly adjacent the edges. Another disadvantage is that the field is defined only at a finite set of points. There is no direct control on how the magnetic field might behave between the points. Further, this technique does not take advantage of the greater efficiency found in z-gradient coils relative to x and y gradient coils.
A technique for designing bunched coils is set forth in U.K. Patent Application No. 2,180,943 of Mansfield, et al. and the corresponding U.S. Pat. No. 4,978,920. This published application provides sets of relationships which describe the induced current density in a superconducting cylinder due to the loops or arcs of segments of current flowing on a smaller diameter cylinder, i.e. the inner coil. However, this technique again fails to consider the inductance or stored energy. Moreover, this technique does not take advantage of the greater efficiency of the z-gradient coil.
Typically, x, y and z-gradient coils are mounted in concentric, bonded layers as illustrated in U.S. Pat. No. 4,713,189 to Punchard, et al. Active shield coils are mounted on three laminated layers of a concentric surrounding cylinder, one drawback of laminating the coils is that there is poor heat dissipation. The coils tend to heat, which can cause distortion of the physical structures and the resultant gradient or shielding magnetic fields.
The present invention contemplates a new and improved self-shielded gradient coil and method for designing such self-shielded gradient coil which overcomes the above-referenced problems and others.